There aren't really any topics that are off-limit per se as long as they are relevant. We encourage debate as long as it is professional and courteous. Contentious issues are fine to discuss as long as everyone involved is contributing positively and the purpose of the discussion is not to put down or minimize anyone. Personal attacks are not tolerated and neither is coming into any forum and posting derogatory material towards that forums members.
There are other guidelines, listed in the TOS, but those are some basic points that hopefully answer your question. Check out this thread as well:
http://forums.studentdoctor.net/showthread.php?t=970068
If anyone has anyone questions about what is appropriate, I encourage you to read the TOS:
http://studentdoctor.net/online-service-agreement/. Please report posts you find inappropriate as opposed to engaging someone who you consider a troll. Feel free to PM any of the SDN moderators if you have other questions.